Developing an in-depth basic understanding of the intrinsic and extrinsic factors that regulate the induction, quality, and longevity of CD8 T cell responses is essential for devising rational strategies to combat viral infections and tumors for which current prevention and treatment regimens are inadequate. Given the biological importance of effector and memory CD8 T cell responses in reducing the morbidity and mortality associated with intracellular infections and tumors, it is paramount to define the factors that determine their functional quality, protective efficacy, and persistence. Although anti-viral CD8 T cells display differences in their portfolios of effector functions, dissecting this ensemble has been impeded by the lack of effective and efficient methods for segregating, tracking, and defining these distinct subsets based upon their cytokine production profiles. Consequently, vital gaps remain in our understanding of how the functional features of anti-viral CD8 T cells become developmentally imprinted, whether they forecast the efficacy of the response, and how these intrinsic properties regulate the establishment of effector, memory, and exhausted populations. Addressing these issues has the potential to transform our understanding of why and how successful protective cellular immunity is elicited by certain natural infections and vaccinations, and why abortive and functionally incompetent responses occur, especially during chronic infections. To overcome the obstacles associated with analyzing distinct CD8 T cell subsets based upon their patterns of cytokine production we have developed and validated double cytokine reporter systems, and have shown that functionally discrete populations are easily detectable and separable using this strategy. This provides a unique and powerful approach for functionally deconstructing the ensemble of anti-viral CD8 T cells during both acute and chronic infections, which has been almost impossible to achieve using conventional techniques. We will now take full advantage of this opportunity to determine the causes and consequences of CD8 T cell functional disparities during the inception (aim 1), as well as effector and memory (aim 2) phases of the response. In addition, we will define the subsets of CD8 T cells that are most susceptible to exhaustion, as well as the functional properties of anti-viral T cells that are more resistant to inactivation and efficacious following immunotherapy (aim 3). Given the ubiquitous roles of CD8 T cells in anti-viral defense our studies are designed to broadly impact the field by defining the common immunological principles that govern the functional heterogeneity of anti-viral CD8 T cells, defining how this is coupled to the immunological and virological outcome of the response, and practically applying this information to promote and reconfigure responses to favor host defense.